It should be understood that the term "light modulating devices" is used in this specification to encompass both light transmissive modulators, such as diffractive spatial modulators, and light emissive modulators, such as conventional liquid crystal displays. Furthermore, in the following description, the term "analogue states" will be used to denote those switching states of the light modulating device which are subject to significant transmission errors, for example because the corresponding transmission levels depend on switching of microdomains in a liquid crystal material. Conversely the term "digital states" will be used to denote those switching states of the light modulating device having substantially error free transmission levels as a result of the fact that such levels depend on a well-defined physical property of the device, for example corresponding to the device being switched fully on or fully off or being in an error free intermediate state as discussed more fully below.
Liquid crystal devices are commonly used for displaying alphanumeric information and/or graphic images. Furthermore liquid crystal devices are also used as optical shutters, for example in printers. Such liquid crystal devices comprise a matrix of individually addressable modulating elements which can be designed to produce not only black and white, but also intermediate tones, or color variations in devices in which color filters are used. The so-called greyscale response of such a device may be produced in a number of ways.
For example, the greyscale response may be produced by modulating the transmission of each element between "on" and "off" states in dependence on the applied drive signal so as to provide different levels of analogue grey. In a twisted nematic device, for example, the transmission of each element may be determined by an applied RMS voltage and different shades of grey may be produced by suitable control of the voltage. In active matrix devices the voltage stored at the element similarly controls the grey level. On the other hand, it is more difficult to control the transmission in an analogue fashion in a ferroelectric liquid crystal device, although various methods have been reported by which the transmission may be controlled by modulating the voltage signal in such a device. In devices having no analogue greyscale, a greyscale response may be produced by so-called spatial or temporal dither techniques, or such techniques may be used to augment the analogue greyscale.
In a spatial dither (SD) technique each element is divided into two or more separately addressable subelements which are addressable by different combinations of switching signals in order to produce different overall levels of grey. For example, in the simple case of an element comprising two equal sized subelements each of which is switchable between a white state and a black state, three grey levels (including white and black) will be obtainable corresponding to both subelements being switched to the white state, both subelements being switched to the black state, and either subelement being in the white state while the other subelement is in the black state. Since both subelements are of the same size, the same grey level will be obtained regardless of which of the subelements is in the white state and which is in the black state, so that the switching circuit must be designed to take account of this level of redundancy. It is also possible for the subelements to be of different sizes which will have the effect that different grey levels will be produced depending on which of the two subelements is in the white state and which is in the black state. However a limit to the number of subelements which can be provided in practice is imposed by the fact that separate conductive tracks are required for supplying the switching signals to the subelements and the number of such tracks which can be accommodated is limited by space constraints.
In a temporal dither (TD) technique at least part of each element is addressable by different time modulated signals in order to produce different overall levels of grey. For example, in a simple case in which an element is addressable by two subframes of equal duration, the element may be arranged to be in the white state when it is addressed so as to be "on" in both subframes, and the element may be arranged to be in the black state when it is addressed so as to be "off" in both subframes. Furthermore the element may be in an intermediate grey state when it is addressed so as to be "on" in one subframe and "off" in the other subframe. The frame rate should be greater than the frequency at which the dither is observable as flickering. Furthermore it is possible to combine such a temporal dither technique with spatial dither by addressing one or more of the subelements in a spatial dither arrangement by different time modulated signals. This allows an increased range of grey levels to be produced at the cost of increased circuit complexity.
In many applications, and particularly in display devices for displaying moving graphic images, there is a requirement for a large number of suitably spaced grey levels to be generated, with minimum (and preferably no) redundancy of grey levels. Usually the grey levels are linearly spaced as far as possible. To this end the elements may be binary weighted, for example by dividing each element into subelements having surface areas in the ratio 1:2:4 in a SD technique or by addressing of each element with frames having durations in the ratio 1:4 in a TD technique. European Patent Publication No. 0261901A2 discloses a method of maximizing the number of grey levels that can be obtained from a certain number of binary temporal divisions of the addressing frame by dividing the addressed rows of the display matrix into groups and addressing the groups sequentially.
European Patent Publication No. 0478043A1 discloses a method of producing a large number of grey levels by combining spatial dither with an analogue switching arrangement so that at least one of the subelements of each element has more than two switching states, that is a black state 0, a white state 1 and at least one intermediate state having a grey level between 0 and 1. For example, each element may be divided into four (column) subelements having widths in the ratio of 4:2:1:1, each of the subelements being switchable between the black state 0 and the white state 1 except for one of the two smallest subelements which is switchable between four analogue states corresponding to 0, 1/3, 2/3 and 1. Taking account of the relative surface areas of the four subelements, it is possible to obtain a total of 32 different grey levels by combining the switching of the four spatial bits with appropriate selection of the different analogue states of the smallest at subelement having the four states 0, 1/3, 2/3 and 1. Provision of such an additional spatial bit having more than two analogue states allows further intermediate grey levels to be produced, and the fact that the spatial bit is a bit of small size means that any errors in the analogue levels are not magnified. However such an arrangement leads to additional circuit complexity and cost, and there are difficulties in manufacturing devices, particularly color display devices, in which a very high density of electrode tracks is required to address the required subelements.
European Patent Publication No. 0361981 discloses a method of maximizing the number of grey levels that can be obtained from a certain number of subpixels in a SD arrangement by dividing each pixel up into n subpixel groups having surface areas in the ratio A1:A2 . . . : An=m.sup.n-1 :m.sup.n-2 :1 where m represents the number of grey levels of each subpixel. Where each subpixel has only two grey levels, that is black and white, and there are three subpixel groups, therefore, the optimized ratio of the surface areas of the subpixel groups is 4:2:1, for example. Different optimized ratios are obtained if each subpixel group has more than two grey levels or if more than three subpixel groups are provided. However such an arrangement may again be limited in its application due to difficulties in manufacturability or manufacturing cost considerations.
W. J. A. M. Hartman, "Ferroelectric liquid crystal displays for television application", Ferroelectrics 1991, Vol. 122, pp.1-26, discloses certain optimum combinations of SD and TD ratios for use in ferroelectric liquid crystal display devices to obtain a large number of spaced grey levels. This reference also describes various methods of achieving different levels of analogue greyscale such as the texture method in which variation in the texture of the liquid crystal material in dependence on the applied electric field is made use of to obtain different grey levels.
Furthermore U.S. Pat. No. 4,712,877 discloses a method of producing discrete grey states within a pixel of a ferroelectric liquid crystal display device by a technique called multi-threshold modulation (MTM), generally by variation of the electric field over the pixel area. For example the liquid crystal thickness may be varied over the pixel area in steps. This method may be combined with dither techniques in order to produce a large number of grey levels, although in practice it is difficult to address more tan a few MTM grey states.
There are a number of inherent physical problems encountered in ferroelectric liquid crystal display devices which result in finite errors in the analogue grey states, and which can accordingly result in unpredictable variation of grey levels with time and/or over the display area. Such problems are discussed in P. Maltese, "Advances and problems in the development of ferroelectic liquid crystal displays", Mol. Cryst. Liq. Cryst. 1992, Vol. 215, pp. 57-72, as well as in K. F. Reinhart, "Addressing of ferroelectric liquid crystal matrices and electrooptical characterisation", Ferroelectrics 1991, Vol.113, pp. 405-417. As is well known, analogue grey states are highly temperature dependent, and the latter reference gives an example in which the display temperature should be uniform to 0.2 degrees if 16 grey levels are required. Both references indicate that the use of tin film transistors for the drive circuitry is advantageous to achieve analogue grey states in such devices.
British Patent Application No. 9603506.8 and Japanese Patent Publications Nos. 27719/1993 and 27720/1993 describe techniques for reducing the error in a 50% analogue grey state to substantially zero by dividing each row (strobe) electrode into two subrows and simultaneously addressing the two subrows such that any local temperature variation has opposite effects in the two subrows tending to cancel the temperature dependence of the grey state for each row. Such a technique allows a substantially error free half (50%) analogue grey state to be obtained. Japanese Patent Application No. 9-72198/1997 describes a technique for obtaining such a substantially error free half state which uses an interlace technique to avoid the need to introduce extra subrows. The term "substantially error free" should be interpreted in this context as meaning tat the error associated with such a state is small by comparison with the errors associated with analogue intermediate grey states produced by conventional means.